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Mistletoe in Oaks
Mistletoe
is a parasite in both the Blue Oak (Quercus douglasii) and
the Valley Oak (Q. lobata). Our oak mistletoe is Phoradendron
villosum, found from California, Arizona, to West Texas. Mistletoe
has both male plants that produce only pollen, and female plants (see
photo) that produce flowers and fleshy, white seed pods. Each pod is filled
with a slimy and sticky clear fluid (ochra anyone?) and one seed covered
with a tough greenish membrane (see photos below). Seeds mature in the
winter.
Various extracts from mistletoe are
being investigated for treating cancer in humans, including ovarian cancer,
lymphoma, and others. However, our mistletoe is very poisonous and should
not be eaten or even nibbled.
Mistletoe fruits are attractive
to wintering birds in the Santa Lucias. Birds can digest most of the outer
white shell of the flower and some of the sticky, clear liquid. When a
bird ingests the seed, the tough outer membrane stays intact.
Seeds
are rapidly defecated by birds (Western Bluebirds, Robins, Cedar Waxwings,
among others) and they still have their slimy, sticky coating. This allows
the seeds to cling to a branch, sprout and insert its root-like "haustoria"
into the water-conducting system of the tree. It takes many years for
the mistletoe to grow large enough to produce flowers and seeds. The haustoria
in mistletoe both penetrate the water-conducting tissue of the trees (water
transport) and infiltrate in between the cells where they absorb most
nutrients. In many populations of mistletoe, the number of male plants
(or female plants) is higher than one would expect (50:50).
Mistletoe can grow into very
large, heavy clumps. But, does it damage the tree? Certainly it makes
some branches more likely to be broken in winter storms, just due to the
increased weight load. In Spain, trees grew at about the same rate with
and without mistletoe. Such experiments have not been done here in California,
yet.
Why are some trees infected with mistletoe,
and others lack mistletoe? In Ponderosa pine, there appear to be chemical
differences in the trees that are infected, suggesting that some may be
simply more resistant. If nothing else, it seems that the chances of being
infected with mistletoe are greatest if a tree is near another tree previously
infected.
Do wildlife use the big clumps
of mistletoe? Yes- there appears to be a greater diversity of birds seen
in trees with mistletoe, and some raptors (birds of prey; Mexican Spotted
Owl and Goshawk) nest preferentially in trees with mistletoe. Some species
of "possums" in Australia are specialists on mistletoe and require
mistletoe, just as pandas require bamboo. Here in the western United State,
martens (native in the weasel family) select mistletoe clumps, often as
shelter from radiation heat loss on clear, cold winter nights.
What controls mistletoe? Apparently
fire can control mistletoe, and I often wonder if the continuous grazing
(fuel removal) and subsequent almost complete lack of fires in the oak
woodlands of California is responsible for the abundance of mistletoe
in many places. Again, experimental fires, repeated frequently, might
tell. Given the new acceptance of fire by the California ranching community,
an opportunity for such a study might arise.
If you had a big oak in your
back yard (or you built your house under a big Blue Oak or Valley Oak)
and you were concerned about a massive infection of mistletoe, what should
you do? Well, there are control options and the UC
Integrated Pest Management Project has some suggestions on a website
but who knows? Several studies suggest that the massive pruning required
to remove the mistletoe will harm the tree. Pruning your oak might make
it more susceptable to harmful insects or fungus. But, if the infestation
is small, you can cut out the mistletoe and then cover the entry point
of the mistletoe with black plastic to kill (no light) the re-emerging
mistletoe stems.
Mistletoe is probably
just another one of the things on a oak that takes 300 years to kill a
big tree.
References (In no Particular Order)
Linhart, Yan B.; Snyder, Marc A.; Gibson, J. Phil. Differential host utilization
by two parasites in a population of ponderosa pine. In: Oecologia (Berlin)
1994. 98 (1): 117-120.
Sterba, H.; Andrae, F.; Pambudhi, F.. Crown efficiency of oak standards
as affected by mistletoe and coppice removal. In: Forest Ecology and Management
. 62 (1-4): 39-49.
Elsaesser-Beile, U.; Voss, M.; Schuehle, R.; Wetterauer, U.. Biological
effects of natural and recombinant mistletoe lectin and an aqueous mistletoe
extract on human monocytes and lymphocytes in vitro. In: Journal of Clinical
Laboratory Analysis 2000. 14 (6): 255-259.
Buchanan, Joseph B.; Irwin, Larry L.; McCutchen, Edwin L.. Characteristics
of spotted owl nest trees in the Wenatchee National Forest. In: Journal
of Raptor Research 1993. 27 (1): 1-7.
Reid, Nick. Control of the mistletoes by possums and fire: A review of
the evidence. In: Victorian Naturalist (Blackburn) 1997. 114 (3): 149-158
Elias, Pavol. A male-based sex ratio in mistletoes. In: Biologia (Bratislava)
1997. 52 (1): 49-51.
Hadfield, James S.; Flanagan, Paul T.. Dwarf mistletoe pruning may induce
Douglas-fir beetle attacks. In: Western Journal of Applied Forestry Jan.,
2000. 15 (1): 34-36.
Overton, Jacob M.. Spatial autocorrelation and dispersal in mistletoes:
Field and simulation results. In: Vegetatio 1996. 125 (1): 83-98.
Lopez-de Buen, Lorena; Ornelas, Juan Francisco. Frugivorous birds, host
selection and the mistletoe Psittacanthus schiedeanus, in central Veracruz,
Mexico. In: Journal of Tropical Ecology May, 1999.
McNamee, D.. Mistletoe extract ineffective in melanoma. In: Lancet (North
American Edition) Sept. 25, 1999. 354 (9184): 1101.
Zarkovic, N.; Zarkovic, K.; Grainca, S.; Kissel, D.; Jurin, M.. The Viscum
album preparation Isorel inhibits the growth of melanoma B16F10 by influencing
the tumor-host relationship. In: Anti-Cancer Drugs 1997. 8 (SUPPL. 1):
S17-S22.
Parks, Catherine G.; Bull, Evelyn L.; Tinnin, Robert O.; Shepherd, Jay
F.; Blumton, Arlene K.. Wildlife use of dwarf mistletoe brooms in Douglas-fir
in northeast Oregon. In: Western Journal of Applied Forestry April, 1999.
14 (2): 100-105.
Hadfield, James S.. Douglas-fir dwarf mistletoe infection contributes
to branch breakage. In: Western Journal of Applied Forestry Jan., 1999.
14 (1): 5-6.
Parks, Catherine G.; Bull, Evelyn L.. American marten use of rust and
dwarf mistletoe brooms in northeastern Oregon. In: Western Journal of
Applied Forestry 1997. 12 (4): 131-133.
Krenzelok, E. P.; Jacobsen, T. D.; Aronis, J. M. (North American Congress
of Clinical Toxicology Annual Meeting Rochester, New York, USA September
16-19, 1995). Mistletoe exposures: The kiss of death? In: Journal of Toxicology
Clinical Toxicology 1995. 33 (5): 543
Ashworth, Vanessa E. (Annual Meeting of the Botanical Society of America
held with the American Institute of Biological Sciences Seattle, Washington,
USA August 3-8, 1996). Phylogenetic relationships within the mistletoe
genus Phoradendron (Viscaceae). In: American Journal of Botany 1996. 83
(6 SUPPL.): 137-138.
Seamans, Mark E.; Gutierrez, R. J.. Breeding habitat of the Mexican Spotted
Owl in the Tularosa Mountains, New Mexico. In: Condor 1995. 97 (4): 944-952.
Calvin, Clyde L.; Wilson, Carol A.. Relationship of the mistletoe Phoradendron
macrophyllum (Viscaceae) to the wood of its host. In: IAWA Journal 1995.
16 (1): 33-45.